This application is related to and incorporates by reference copending applications Ser. No. 055,166 titled "Proximity Detector Employing Sequentially Generated Mutually Orthogonally Polarized Magnetic Fields", and Ser. No. 055,164 titled "Proximity Monitoring Apparatus Employing Encoded, Sequentially Generated, Mutually Orthogonally Polarized Magnetic Fields" both filed Apr. 30, 1993, both assigned to the assignee hereof, and both by the same inventor, Donald K. Belcher, one of the inventors hereof.
The present invention is directed to devices and methods for determining the proximity of a radio transmitter, and more particularly to a device and method for determining the proximity of a radio transmitter to a radio receiver that has a sharp, repeatable threshold of detection in diverse electromagnetic environments.
Proximity detection devices are used in a wide variety of applications for determining the relative nearness or separation of an object or person relative to another object or person. An application of such devices that has recently acquired considerable public interest involves using such devices to allow a responsible individual, such as a parent or guardian, to monitor the whereabouts of another person, such as a child, in the custody of the responsible individual. Typically, proximity monitoring is performed by equipping each of a parent and child with a respective radio transmitter/radio receiver pair. The radio transmitter carried by the child radiates a radio frequency (RF) electromagnetic signal to which the radio receiver in the unit carried by the parent is tuned. Should the level of a received electromagnetic signal monitored by the parent's receiver drop below a prescribed threshold, indicating that the child has moved beyond a prescribed distance from the parent, an alarm signal is generated by the parent's device.
Such devices have various drawbacks, not the least of which is that electromagnetic radio wave transmissions are subject to multipath propagation, which can be especially severe in the interior of a building. A further problem is the effect of dielectric distortion of the human body on the signal, which can cause a fluctuation in the signal amplitude on the order of 10-15 dB. Moreover, since the strength of a radiated electromagnetic signal is inversely proportional to the square of the distance from the emitter, the setting of a signal strength threshold to trigger an alarm yields very imprecise results, especially when considering the other effects described above.
These problems are addressed, in part, by the abovedescribed copending applications. Those applications describe a magnetic field-based system in which a plurality of relatively low frequency (tens to hundreds of kilohertz) magnetic fields, having mutually orthogonal polarizations, are used. The magnetic fields are generated in a time sequence that is modulated by a prescribed encoding pattern, so as to give each pair of transmitter and receiver a unique identity and permit multiple systems to be used in the same operating environment without mutual interference. The magnetic field-based proximity detector system includes a magnetic field generator which is provided within a first device, as may be carried by an object, animal, or person whose whereabouts is being monitored. The magnetic field generator sequentially generates a plurality of magnetic fields of respectively different magnetic field polarizations. The magnetic field generator sequentially generates three time-varying magnetic fields, each having respective field polarizations that are mutually orthogonal to one another, so that complete coverage, without nulls, is provided for a three dimensional space coordinate system.
A magnetic field sensor is provided within a second device carried by another individual such as a parent or guardian monitoring the whereabouts of the animal, object, or person carrying the first device. The magnetic field sensor detects magnetic field energy associated with one or more of the encoded magnetic fields sequentially generated by the magnetic field generator carried by the monitored individual. The magnetic field sensor unit preferably includes one or more magnetic field sensors having respective magnetic field polarization sensitivities that are oriented mutually orthogonal with respect to each other. The outputs of the magnetic field sensors are coupled to respective decoder circuits that compare received sensor signals with a stored code pattern corresponding to that employed by the first device. Whenever the coded contents of the received signals match the stored code, a hit is declared by the decoder circuit associated with each respective magnetic field polarization channel. The output from each decoder is coupled to a timeout circuit, which monitors the rate at which it is receiving hits from the outputs of any of the decoders. As long as a hit is received from any decoder within a prescribed time interval, a determination is made that the monitored object is within the proximity of the monitoring individual. However, if the monitored object goes out of range, none of the magnetic field sensors will detect sufficient energy to permit a code match hit to be declared. When the timeout circuit receives no hit within the required time interval, it generates an output signal indicating that the first device is no longer in the proximity of the second device.
Magnetic fields provide the advantage that their power density with respect to distance has a very emphatic inverse proportionality characteristic, namely, it is inversely proportional to the sixth power of the distance. Accordingly, the slope of the magnetic field signal strength variation is extremely steep over the major portion of the working range of the receiver, thereby allowing an out-of-range threshold to be readily and accurately established. In addition, a magnetic field does not radiate in the manner of a radio frequency electromagnetic wave, therefore the previously described problems of multi-path propagation and human body dielectric distortion are not present.
However, even the elegant use of the magnetic field described above does not adequately address the problem of providing such a system in a low cost and energy efficient manner. For example, it is desirable that the time sequences of generated magnetic waves be modulated and subsequently demodulated in a simple system. Further, it is desirable that the system consume low levels of energy.
The generation of the magnetic field with a frequency within the range of tens to hundreds of kilohertz presents further problems in that there are a number of sources of potentially interfering radiation in this range. For example, incidental fields emanate from consumer devices such as televisions and personal computers, the government operates a chain of high-powered transmitters at 100 Khz to support LORAN, and televisions operate with a horizontal scan oscillator operating at 15,750 Hz with harmonics of the signals being quite severe.
Further, the codes used by each pair of transmitter and receiver may contain a number of similar sequences so that missed portions of the code may lead to misidentification. Accordingly, fit is desirable to provide a group of codes that have a high degree of orthogonality between codes and time-skewed versions of different codes (that is, the group of codes that includes codes that are not likely to be confused with each other).
Accordingly, it is an object of the present invention to provide a novel system and method of determining whether a first device is approximate a second device that obviates the problems of the prior art.
It is another object of the present invention to provide a novel device and method for determining whether a first device is approximate a second device that uses a plurality of time-sequential magnetic fields that are differential phase shift key (DPSK) modulated codes. DPSK modulation provides the advantages of simplified demodulation circuitry and the absence of the need for gain control in the receiving unit.
It is yet another object of the present invention to provide a novel device and method of determining whether a first device is approximate a second device in which a magnetic field is transmitted at a frequency that obviates interference from known sources of radiation.
It is another object of the present invention to provide a novel method and device of determining whether a first device is approximate a second device in which a magnetic field is DPSK modulated in a code selected from a predetermined group of generally orthogonal codes.